ClassVar breaks dataclass fields

[michael-123123]

Bug report

Bug description:

Hi,

I searched issues and couldn't find anything similar to this.
I'm not 100% sure it's a bug - possibly this is desired behavior?

To reproduce:

from dataclasses import Field, field, dataclass
from typing import ClassVar


class A:
    sentinel: ClassVar[Field] = field()


@dataclass
class B:
    sentinel: ClassVar[Field] = field()


print("Class A")
print(A.sentinel)
print(A().sentinel)
print()


print("Class B")
try:
    print(B.sentinel)
except AttributeError as ex:
    print(ex)

try:
    print(B().sentinel)
except AttributeError as ex:
    print(ex)

print()

Output:

Class A
Field(name=None,type=None,default=<dataclasses._MISSING_TYPE object at 0x78ec801c1010>,default_factory=<dataclasses._MISSING_TYPE object at 0x78ec801c1010>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),kw_only=<dataclasses._MISSING_TYPE object at 0x78ec801c1010>,_field_type=None)
Field(name=None,type=None,default=<dataclasses._MISSING_TYPE object at 0x78ec801c1010>,default_factory=<dataclasses._MISSING_TYPE object at 0x78ec801c1010>,init=True,repr=True,hash=None,compare=True,metadata=mappingproxy({}),kw_only=<dataclasses._MISSING_TYPE object at 0x78ec801c1010>,_field_type=None)

Class B
type object 'B' has no attribute 'sentinel'
'B' object has no attribute 'sentinel'

Upon inspection of B.__dataclass_fields__ the sentinel field does appear in B.
Which is somewhat confusing behavior.

CPython versions tested on:

3.13

Operating systems tested on:

Linux

[ericvsmith]

It's not clear what you're trying to accomplish.

__dataclass_fields__ is private. Use dataclasses.fields() and you'll see it's not a normal field. You've not initialized the class field to anything.

This is the normal way to initialize a ClassVar.

>>> @dataclass
... class B:
...     sentinel: ClassVar = 3

[michael-123123]

Hi @ericvsmith.
Thanks for the quick reply.

I realize that.
Let me clarify that I wanted to have a class variable that is of type Field.

The point I was making with the bug report is that if you want a ClassVar of type Field --
then it is neither a class var nor an attribute of the instance.

This may be some kind of undefined/undocumented behavior.
I would expect this to just be a regular class variable of type Field (like in class A in my example)
Alternatively - I would expect some error message that I have an uninstantiated field - similar to non class var fields that are not initialized.

[michael-123123]

Actually I am closing this bug -- this behavior is documented.
The workaround (in case anyone is interested) is:

@dataclass
class B:
    x: ClassVar[Field] = field(default=field())

[ericvsmith]

Wouldn't this do the same thing?

@dataclass
class B:
    x: ClassVar[Field] = field()

Although maybe this would be better:

@dataclass
class B:
    x: ClassVar[Field] = field(default_factory=field)

[michael-123123]

Hi @ericvsmith
The point of my original post was that the first example you give is not equivalent to the second.

@dataclass
class A:
    x: ClassVar[Field] = field()

doesn't work (crashes with exception).

But,

@dataclass
class B:
    x: ClassVar[Field] = field(default=field())

does work (as expected x is an attribute of class B with type Field).

I guess the idea is that the field value should be "escaped" by another field.